To get a 5%
reduction in time, a robot may be initiated by a sound rather than a manual
action. The rules for the Trinity contest define this as a 3 to 4 KHztone
of which the Radio Shack #273-075 piezo buzzer is an example.

The circuits below cover the
sound generator itself; and the detection of the sound and output of a signal to
input to the robotís processor.

The tone generator is easily built by connecting a 9 volt battery to the
buzzer through a momentary switch. I believe the unused blue wire was to
give a pulsing tone.

The detector (below) is a bit more complex. It amplifies a microphone
and sends the resulting signal to an NE567 tone decoder.

The amplifier is half of a 1458 opamp. The two 120K resistors attached
to pin 3 keep the opamp's average output at about halfway between the 5 vdc
source and ground. These resistor values are non-critical, but should be
equal.

The 120K feedback resistor from pin 1 to 2 establishes the gain of the
amplifier and the sensitivity of the detector to a generated tone. I found
that 120K gave me pretty reliable operation at about with the generator about 3
feet from the microphone. You can vary this resistor to make the detector
more or less sensitive if you wish. More resistance makes it more
sensitive.

The two 0.01 uf capacitors are just to eliminate dc voltages. Their
values are non-critical.

The 567 works by comparing the input signal to an internal oscillator whose
frequency is determined by the 25K variable resistor and the 0.022 uf
capacitor. You vary these components as necessary to get the oscillator at
the same frequency as your tone generator. I used a 10 turn pot for the
25K which worked fine. The resulting resistance to match my tone generator
was about 11K. The 567 requires a pretty close frequency match to trip
(the output goes low when the frequencies match). I find that I
occasionally have to readjust the 25K resistor to get a reliable trip. You
might want to make this adjustment accessible if you package the circuit.

The 20K pullup resistor on the output can be changed or deleted to match your
processor's requirements.

The 1uf and 0.1uf capacitors provide some filtering and bandwidth
adjustment. See the application notes if you want to adjust them.